1. Field of the Invention
The present invention generally relates to a light emitting device and a method for producing the same. More specifically, the invention relates to a light emitting device wherein a light emitting element is combined with a phosphor (or a fluorescent substance) which is designed to receive light from the light emitting element to emit light having a different wavelength from that of the light emitted from the light emitting element, and a method for producing the same.
2. Description of the Prior Art
In recent years, there has been actively developed a light emitting device for emitting white light by combining a light emitting element, such as a light emitting diode (LED) for emitting light in wavelength ranges from near ultraviolet to blue, with a phosphor (or a fluorescent substance) which is designed to receive light from the light emitting element to emit light having a different wavelength from that of the light emitted from the light emitting element. In such a white light emitting device, a LED chip is mounted on the bottom face of a cup-shaped recessed portion formed in a lead frame or substrate, and a resin mixed with a phosphor, which is designed to absorb light from the LED chip to emit light having a longer wavelength than that of the light from the LED chip, is potted in the cup-shaped recessed portion to seal the LED chip therein.
If such a white light emitting device is used as a light source for illumination, it is desired that the device can emit light having a high emission power (a high illumination intensity or illuminance), a small irregularity in color, and good color rendering properties. In order to cause light from such a white light emitting device to be white light having good color rendering properties, it is proposed that a resin mixed with two kinds of phosphors for emitting light having different wavelengths by light from a LED chip is arranged around the LED chip (see, e.g., Japanese Patent Laid-Open Nos. 2000-244021, 2001-127346, 2003-101081, 2003-318447 and 2004-152993). In addition, there is proposed a light emitting device for emitting white light by arranging a resin mixed with a YAG phosphor (a rare-earth aluminate phosphor activated by a rare earth element having a garnet structure), which is designed to emit light in wavelength ranges from green to yellow as a complementary color light with respect to light in the blue range, around a light emitting element which is designed to emit light in the blue range. There is also proposed a white light emitting device capable of realizing a luminescent color having higher color rendering properties by mixing a resin with a phosphor of Ca2Si5N8:Eu or the like which is designed to emit light in wavelength ranges from orange to red (see, e.g., WO2005/029596).
In a white light emitting device wherein a light emitting element is mounted on the bottom face of a cup-shaped recessed portion, which is formed in a lead frame or substrate, to be sealed with a resin containing phosphor particles by potting the resin, it is required that the optical path length, by which light emitted from the light emitting element passes through the resin, is constant in order to reduce the irregularity in color. In order to make it constant, it is required that the top face of a sealing member of a resin is raised to be convex so that the distance between the light emitting element and the surface of the sealing member is constant.
However, when the light emitting element arranged in the cup-shaped recessed portion is sealed with a resin by potting, it is not easy to control the shape of a sealing member of the resin so that the optical path length, by which light emitted from the light emitting element passes through the resin, is constant, due to the viscosity of the used resin and the gravity applied to the resin. As a result, the irregular color of light from the light emitting device is caused.